The use of porcelain compositions in the field of dentistry is well known, as are porcelain compositions themselves and procedures for preparing porcelain compositions. See, for example, U.S. Pat. No. 3,423,828 to Halpern, which discloses a synthetic resin denture base comprising a major portion of porcelain particles; U.S. Pat. No. 3,464,837 to McLean, which discloses a porcelain-containing material that is suitable for use as a dental enamel veneer; and U.S. Pat. No. 3,052,982 to Weinstein, which relates to fused porcelain-to-metal teeth. Similarly, porcelains, including dental porcelains, which contain some quantity of leucite are known. See, for example, 82 Chemical Abstracts 63365m, which relates to leucite glass-ceramic enamel compositions which may be bonded to gold alloy substrates; and U.S. Pat. No. 4,101,330 to Burk, which discloses the preparation of a porcelain or ceramic body from a ceramic raw material consisting essentially of at least about 78.1 percent by weight nepheline syenite, about 3 to 7 percent by weight of pre-formed leucite particles and about 0 to 15.5 percent by weight of at least one modifier selected from the class consisting of oxides and oxide precursors of potassium, sodium, and lithium. The ceramic raw materials are such that when fused to form a fired ceramic body, the resulting body comprises a modified nepheline syenite glassy phase having leucite particles dispersed therein.
Other prior art which relates generally to dental and/or porcelain compositions, some of which may contain leucite crystals, include U.S. Pat. No. 4,120,729 to Smyth, entitled "Novel Low Temperature Maturing Dental Glaze;" 81 Chemical Abstracts 81713g, an abstract entitled "Enamels with High Thermal Expansion Doefficient;" U.S. Pat. No. 4,337,316 to Votava, entitled "Sanitary Ware and Process of Production;" U.S. Pat. No. 3,775,164 to Smith, entitled "Method of Controlling Crystallization of Glass;" and U.S. Pat. No. 3,615,765 to Bystrova, entitled "Glaze for Ceramic Parts and Articles."
Dental porcelains generally may be classified as higher fusing porcelains, i.e., those porcelains fusing above about 1000.degree. C., and lower fusing porcelains, i.e., those porcelains fusing below about 1000.degree. C. The higher fusing porcelains generally exhibit resistance to thermal stress and mechanical shock and to erosion by mouth fluids, and have been fused to metals having a compatible thermal coefficient of expansion, such as the platinum-iridium alloys. The lower fusing porcelains have been used for using to lower melting substrates, such as gold alloys, but there have been problems, at least in part, because of the disparity between the thermal coefficient of expansion of the gold alloys and the lower fusing porcelains.
Attempts have been made to match a given porcelain material to a given dental support metal so as to increase the compatibility between their respective thermal coefficients of expansion. These attempts have included the preparation of specifically formulated and prepared porcelains to be used with dental metals having a specific narrow thermal expansion range. For example, in U.S. Pat. No. 3,052,982 to Weinstein, there is disclosed a technique for preparing porcelain coverings for metal supports, wherein the porcelain is tailored in each case to have a thermal coefficient of expansion which is compatible with that of the metal being used for the support. The porcelains disclosed in this patent are prepared by mixing predetermined amounts of components prepared in part from different feldspars and glasses, the composition and relative amounts of the components being responsible for the physical characteristics of the porcelain products.
While techniques of the above type have been used with some success in the art, they often result in porcelains which have expansion characteristics that vary with firing conditions.
Accordingly, it is an object of the present invention to provide dental porcelain compositions which, when fused, provide a porcelain body having controlled coefficient of thermal expansion.
It is another object of the invention to provide dental porcelain compositions which selectively and readily can be tailored for use with a variety of commercially available metal support materials.
Still another object of the invention is to provide an efficient technique for matching the coefficient of thermal expansion of a dental porcelain composition with that of a metal support structure to which the porcelain composition is to be fused.
Another object of the invention is to provide a dental porcelain material having all the characteristics necessary for advantageously producing an aesthetic dental restorative.
Yet another object of the invention is to provide a unique system of leucite-containing raw materials which can be blended and fused to form a dental porcelain having a preselected coefficient of thermal expansion.
Still another object of the invention is to provide a facile technique for providing a range of porcelain materials, each having a preselected coefficient of thermal expansion and each having a firing temperature on the order of about 700.degree. C. to about 1315.degree. C.
Yet another object is to enable the modification of a naturally occurring potash feldspar or glass of the same composition to form a series of thermal expansion control, leucite-containing frits.
Still another object is to control the coefficient of thermal expansion of a dental porcelain through the use of porcelain-forming raw materials comprised of intentionally graded leucite-containing frits prepared from a feldspar material.
These and other objects and advantages of the present invention are achieved, in a broad sense, by first providing a series of leucite-containing, glass-ceramic frits, each of the series containing a different amount of leucite and thus exhibiting a different coefficient of thermal expansion. The series of glass-ceramic frits can be blended selectively with a matrix glass to control the thermal expansion coefficient of the glass matrix/glass-ceramic blend. The matrix glass in combination with the glass-ceramic frits is selected to govern the firing temperature, glass transition temperature, viscosity, and translucency of the resulting system.
In one aspect of the invention, the glass matrix is comprised of a mixture of two glasses. Preferably, one of the glasses would be a relatively higher melting glass having a fusion range on the order of from about 750.degree. C. to about 845.degree. C., preferably 800.degree. C. to about 830.degree. C., and the other would be a relatively lower melting glass having a fusion range in the range of from about 700.degree. C. to about 825.degree. C., preferably 785.degree. C. to about 815.degree. C. In most cases, the difference in fusion range between the lower melting glass and the higher melting glass is at least about 10.degree.-15.degree. C. The mixture or glasses generally would possess the desired balance between fluidity and stiffness, and surface properties attributable to the two glasses comprising the mixture. The glasses also would be selected such that the refractive index of the fusion product of their mixture would be about the same as that of the leucite-containing glass-ceramic frits, and such that the coefficient of thermal expansion of their fusion products would be in the range of from about 9.times.10.sup.-6 to about 10.times.10.sup.-6 in/in/.degree.C. at 400.degree. C. Although a number of glasses satisfy the above selection criteria, suitable examples of matrix glasses are shown in Table 1.
TABLE 1 ______________________________________ Higher Melting Lower Melting Matrix Glass Matrix Glass Constituents Percent by Weight Percent by Weight ______________________________________ SiO.sub.2 67.0-73.0 61.0-66.0 Al.sub.2 O.sub.3 7.2-9.3 12.3-14.8 CaO 0.2-1.0 2.3-4.5 Na.sub.2 O 12.5-14.4 8.5-10.2 K.sub.2 O 6.2-7.1 5.0-5.9 Sb.sub.2 O.sub.3 0.1-0.3 0.1-0.35 Fe.sub.2 O.sub.3 0.1 0.1 PbO 0.1 0.1 Li.sub.2 O.sub.3 1.5-3.5 BaO 0.5-1.0 B.sub.2 O.sub.3 0.2-0.6 MgO 0.8-2.0 ______________________________________
Thus, in one aspect of the invention the relatively higher melting glass matrix material may comprise a frit having a composition consisting essentially of, by weight percent, about 67.0 to 73.0 percent silicon dioxide, 7.2 to 9.3 percent aluminum oxide, 0.2 to 1.0 percent calcium oxide, 12.5 to 14.4 percent sodium monoxide, and 6.2 to 7.1 percent potassium oxide, and 0.1 to 0.3 percent antimony(III) oxide, and the second or relatively lower melting glass matrix material may comprise a frit having a composition consisting essentially of by weight percent, about: 61.0 to 66.0 percent silicon dioxide, 12.3 to 14.8 percent aluminum oxide, 2.3 to 4.5 percent calcium oxide, 8.5 to 10.2 percent sodium monoxide, 5.0 to 5.9 percent potassium oxide, 1.5 to 3.5 percent lithium oxide, 0.5 to 1.0 percent barium oxide, 0.2 to 0.6 percent boric oxide, 0.8 to 2.0 percent magnesium oxide, and 0.1 to 0.35 percent antimony (III) oxide.
Generally speaking, the matrix glasses may be mixed over a fairly wide range of higher to lower melting glass. However, it is normally preferred that the higher to lower melting glass be mixed in a ratio of from about 1:10 to 10:1 with ratios of from about 1:2 to 2:1 being most suitable.
The leucite-containing, glass-ceramic fritted materials which are to be blended with the matrix glass are themselves comprised of a glassy phase matrix in which is dispersed particles of leucite crystals. The series of leucite-containing frits are graded in the sense that each frit contains a different amount of leucite particles and thus a different coefficient of thermal expansion. For example, it is contemplated that a first member of the series might contain 10 percent by weight leucite, that a second member might contain 20 percent by weight leucite, that a third member might contain 40 percent by weight leucite, and so on, with the balance of each member being the glassy phase matrix. It is an important feature of the present invention that the glassy phase matrix can be derived from a feldspathic material which is the same feldspathic material that is used to prepare each member of the series. As used in this specification and claims, the term feldspathic material is meant to describe naturally occurring feldspars as well as glasses and mixtures of oxides having substantially the same chemical composition as such naturally occurring feldspars.
In one embodiment of the invention, the series of leucite-containing, glass-ceramic frits is prepared by doping or adding potassium nitrate, potassium carbonate, potassium silicate, or some other equivalent potassium sorce to a feldspathic material such as a potash feldspar, in separate batches, adding a different amount of potassium nitrate to each separate batch. The potassium nitrate-doped batches are then heated in a furnace at a temperature of from about 1120.degree. C. to about 1650.degree. C. or potentially as high as the equipment will allow for a period up to about eight hours. During this heating period, all of the feldspar melts and leucite crystals begin to precipitate. The leucite particles generally are in the micron size range, for example, on the order of about 2 to 50 microns. The resulting frit is cooled and pulverized to a size on the order of -200 mesh for admixture with the matrix glasses described above.
In another embodiment, a frit is prepared from an undoped potash feldspar using the same procedure outlined above. except that no source of potassium is added to the feldspar. The resulting frit, which may be referred to as a zero-doped frit, may be mixed with the matrix glasses described above.
The composition of a series of leucite-containing, glass-ceramic frits prepared in accordance with the invention may vary over relatively wide limits with respect to the amount of leucite contained therein. However, since all of the members of the series are prepared by doping a feldspathic material in a systematic fashion, the overall composition of the various members of the series will be essentially the same, except for the ratio of leucite to residual glass. In addition, those members of the series having higher percentages of leucite will have correspondingly higher coefficients of thermal expansion. It is this difference in thermal expansion which enables the use of the compositions of this invention to prepare dental porcelains having controlled expansion characteristics. The composition of a series of leucite-containing glass-ceramic frits prepared in accordance with a preferred aspect of the invention is shown in Table 2. Also shown in Table 2 is the stoichiometric composition of leucite and a frit prepared only from a potash feldspar and free from detected leucite. Table 3 illustrates the coefficient of thermal expansion of the frits obtained by doping the potash feldspar shown in Table 2 with 0, 2, 4, 6, 9, and 11 percent by weight potassium nitrate, respectively, based on the total composition. It will be appreciated that other potassium nitrate dopant levels or the use of other potassium sources may be used in accordance with this invention as well. It will be appreciated, also, that feldspathic materials other than a naturally occurring potash feldspar may be used.
TABLE 2 __________________________________________________________________________ Potassium Nitrate Doped Feldspar Series All Constituents in % by Weight Composition SiO.sub.2 Al.sub.2 O.sub.3 FeO CaO Na.sub.2 O K.sub.2 O Other __________________________________________________________________________ Potash feldspar 64.80 18.85 0.01 0.05 3.20 12.92 0.17 Potash Feldspar + 2% KNO.sub.3 64.19 18.67 0.01 0.05 3.17 13.74 0.17 Potash Feldspar + 4% KNO.sub.3 63.57 18.50 0.01 0.05 3.14 14.57 0.16 Potash Feldspar + 6% KNO.sub.3 62.93 18.31 0.01 0.05 3.11 15.43 0.17 Potash Feldspar + 9% KNO.sub.3 61.95 18.02 0.01 0.05 3.06 16.76 0.16 Potash Feldspar + 11% KNO.sub.3 61.25 17.84 0.01 0.04 3.02 17.67 0.16 Stoichiometric Leucite 55.10 23.30 21.60 __________________________________________________________________________
TABLE 3 ______________________________________ Frit Thermal Expansion Data (.times. 10.sup.-6 in/in/.degree.C.) TE @ 25.degree. C. Composition 400.degree. C. 500.degree. C. 600.degree. C. ______________________________________ *Potash Feldspar 15.13 15.31 15.44 *Potash Feldspar + 2% KNO.sub.3 14.7 15.83 15.44 14.32 15.53 14.92 **Potash Feldspar + 4% KNO.sub.3 16.62 18.55 17.58 16.65 18.64 18.77 16.76 18.72 -- *Potash Feldspar + 6% KNO.sub.3 17.19 20.21 20.77 **Potash Feldspar + 9% KNO.sub.3 16.21 18.51 23.16 **Potash Feldspar + 11% KNO.sub.3 16.35 18.52 24.56 16.48 18.63 24.98 ______________________________________ *Fired in Research Kiln **Fired in Factory Kiln
In one preferred embodiment, a porcelain material, which is suitable for fusion to a metal substrate, is prepared using a two component glass matrix and two or more, but most preferably two, leucite-containing, glass-ceramic frits to control the thermal expansion. The two component glass matrix, which is comprised of a higher melting glass and a lower melting glass, as described above, is blended into separate batches with the two or more glass-ceramic frits to form a series of master frits containing different amounts of leucite crystals. Each master frit thus would be prepared from the two components of the matrix glass and one of the glass-ceramic frits, including, if desired, a zero-doped frit. The porcelain material would then be made by blending together a mixture of two or more master frits. Accordingly, in the case where two master frits are employed, the resulting porcelain would comprise four components, namely: the two glasses comprising the matrix glass and two leucite-containing, glass-ceramic frits. The selection of the master frits would be made so as to obtain the desired coefficient of thermal expansion in the final porcelain. Generally speaking, the resulting porcelain materials selectively would exhibit a coefficient of thermal expansion of from about 10 to about 19 in/in/.degree.C. at 500.degree. C. This would enable a manufacturing technician to prepare quite readily from a stock of a relatively few master frits porcelain materials that would be suitable for use with any dental alloy.
The master frits may comprise a majority of glass matrix material or a majority of glass-ceramic frit as is desired. However, the amount of glass matrix to glass-ceramic used to prepare the master frits generally is in the range of from about 1:10 to 10:1 by weight, and preferably is in the range of from about 1:2 to 2:1.
In another embodiment, the porcelain material can be prepared simply by blending and then firing a mixture of the two glass components comprising the glass matrix and two or more, but preferably two, glass-ceramic frits of different leucite content. Again, if desired, one of the glass-ceramic frits may be prepared from a zero-doped feldspar or from another suitable potassium-doped or zero-doped feldspathic material. In this embodiment, the amounts of the various components generally would be such that the ratio of higher melting to lower melting glass of the glass matrix would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1; the ratio of the two different glass-ceramic frits would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1; and the ratio of the glass matrix (both glasses combined) to the glass-ceramic frit material (all glass-ceramic frit components combined) would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1. Generally speaking, the glass matrix (both glasses combined) will comprise from about 20 to about 80 percent by weight of the total weight of the various components.
It will be appreciated that the present invention enables the preparation of a wide variety of dental prosthetic devices which require a porcelain veneer to be bonded to a metal support, usually through an opaque layer. These devices which include, for example, fixed or removable bridgework, crowns, and the like can be prepared with relative ease and exceptional reproducibility with respect to matching the coefficient of thermal expansion of the porcelain veneer material with that of the metal support.